Wash the prawns and shell them without removing the tails. Pour the water into a pan.
Add lemon grass, kaffir lime leaves and galangal root. Boil for 5 minutes.
Add mushrooms, fish sauce, lime juice, milk, Tom Yum paste and simmer for 2 minutes or until mushrooms are tender.
Add prawns and cook until they turn pink.
Finish by adding Thai bird chilies to taste and garnish with fresh cilantro leaves.